1. Field of the Invention
The present invention relates to a semiconductor stress sensor comprising a field-effect transistor formed on a semiconductor substrate of GaAs (gallium arsenide) or the like, for detecting a stress applied to the field-effect transistor.
2. Description of the Relevant Art
It is known in the art from Japanese laid-open patent publication No. 61-153537, for example, that when a stress is applied to a field-effect transistor made of a piezoelectric semiconductor such as GaAs, the drain current of the field-effect transistor varies. There have been used semiconductor stress sensors for detecting external forces such as pressure, accelerations based on the above nature of a field-effect transistor.
FIG. 5 of the accompanying drawings shows the circuit arrangement of a conventional semiconductor stress sensor.
The conventional semiconductor stress sensor, generally designated by the reference numeral 101 in FIG. 5, comprises a field-effect transistor 102 made of a piezoelectric semiconductor such as GaAs or the like, a current-to-voltage (I-V) converter 103 for producing a voltage output corresponding to a drain current ID from the field-effect transistor 2, and a gate bias voltage generator 104 composed of two series-connected resistors R1, R2 for producing a gate bias voltage to be applied to the field-effect transistor 2.
The field-effect transistor 102 has a drain D connected to a power supply VDD and a gate G connected to the junction between the resistors R1, R2. Therefore, the voltage divided by the resistors R1, R2 is applied as a gate bias voltage VG to the gate G of the field-effect transistor 102.
The current-to-voltage converter 103 comprises an operational amplifier 103a and a feedback resistor 103b. The operational amplifier 103a has an inverting input terminal 103c connected to the source S of the field-effect transistor 2, and a non-inverting input terminal 103d connected to ground. The feedback resistor 103b is connected between the output terminal of the operational amplifier 103a and the inverting input terminal 103c. The potential difference between the input terminals 103c, 103d of the operational amplifier 103a is substantially zero, so that the potential at the inverting input terminal 103, i.e., the source potential of the field-effect transistor 102, is substantially at the ground level. Therefore, the field-effect transistor 102 is driven with a constant voltage.
When no stresses are detected by the semiconductor stress sensor 101, i.e., when the semiconductor stress sensor 101 is in a standby mode, the voltage from the power supply VDD is cut off, or the power supply VDD is switched off, to cut off the drain current ID of the field-effect transistor 102. In the standby mode, therefore, any current which is consumed by the semiconductor stress sensor 101 is reduced.
However, since the drain D, gate G, and source S of the field-effect transistor 102 are de-energized in the standby mode, it will take several seconds to several minutes for the semiconductor stress sensor 101 to be able to produce a stable detected output signal after the power supply voltage starts to be applied for stress detection. For a certain period of time after the power supply voltage starts to be applied, even if a constant stress is applied to the field-effect transistor 101, the detected output signal tends to vary gradually or the drain current ID is liable to increase or decrease for several seconds to several hours. Such a drift should be avoided as much as possible.
To effect highly accurate stress measurements, therefore, the semiconductor stress sensor 101 should be energized for a sufficient period of time before it is required to detect applied stresses. As a result, the standby mode of the semiconductor stress sensor 101 is available only for short periods of time, and hence the power consumption by the semiconductor stress sensor 101 cannot be reduced.